There are many aspects to "modern" bikes, involving both improvements and change for change's sake. Aside from those, there is also conservatism for conservatism's sake, and prejudice against previously rejected developments that may have been rejected for bad reasons. Taken all together, a return to such things as internally geared hubs and leather saddles is not necessarily throwback, but may in fact reflect a sense that a 'new' technology did not live up to its promise. A leather saddle is not necessarily a throwback, though the current Brooks saddles (that replicate the pre-1954 round badge and skirt stamps) clearly are a throwback. There are more modern leather saddles, but they, too, seem modeled on very old designs.

Where conservatism is most obvious is wheel size and rider position. Bicycle designers of the 1880's developed the "safety bicycle" and picked a wheel size that has hardly changed since then. Rider position has hardly changed either. Smaller wheels have advantages --see Moulton-- but there is an inherent prejudice against them. I can't tell you how many times I've heard someone ask if the advantages of smaller wheels are enough to overcome the disadvantages. On investigation the disadvantages are mostly imaginary or aesthetic; but that doesn't matter. People want a modern bike, but not too modern.

If you want the fastest, most modern bike design, with no compromises, you'll get something like this: